Renal physiology (Latin renes, "kidneys") is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassium, and other electrolytes; clearance of toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D. Much of renal physiology is studied at the level of the nephron, the smallest functional unit of the kidney. Each nephron begins with a filtration component that filters the blood entering the kidney. This filtrate then flows along the length of the nephron, which is a tubular structure lined by a single layer of specialized cells and surrounded by capillaries. The major functions of these lining cells are the reabsorption of water and small molecules from the filtrate into the blood, and the secretion of wastes from the blood into the urine. Proper function of the kidney requires that it receives and adequately filters blood. This is performed at the microscopic level by many hundreds of thousands of filtration units called renal corpuscles, each of which is composed of a glomerulus and a Bowman's capsule. A global assessment of renal function is often ascertained by estimating the rate of filtration, called the glomerular filtration rate (GFR).
Formation of urine
The kidney's ability to perform many of its functions depends on the three fundamental functions of filtration, reabsorption, and secretion, whose sum is called renal clearance or renal excretion. That is:
Urinary excretion rate = Filtration rate – Reabsorption rate + Secretion rate Although the strictest sense of the word excretion with respect to the urinary system is urination itself, renal clearance is also conventionally called excretion (for example, in the set term fractional excretion of sodium).
Filtration
The blood is filtered by nephrons, the functional units of the kidney. Each nephron begins in a renal corpuscle, which is composed of a glomerulus enclosed in a Bowman's capsule. Cells, proteins, and other large molecules are filtered out of the glomerulus by a process of ultrafiltration, leaving an ultrafiltrate that resembles plasma (except that the ultrafiltrate has negligible plasma proteins) to enter Bowman's space. Filtration is driven by Starling forces.
The ultrafiltrate is passed through, in turn, the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and a series of collecting ducts to form urine.
Reabsorption
Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. It is called reabsorption (and not absorption) both because these substances have already been absorbed once (particularly in the intestines) and because the body is reclaiming them from a postglomerular fluid stream that is well on its way to becoming urine (that is, they will soon be lost to the urine unless they are reclaimed). Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium (the connective tissue that surrounds the nephrons), and then the transport of these substances from the interstitium into the bloodstream. These transport processes are driven by Starling forces, diffusion, and active transport.
Indirect reabsorption In some cases, reabsorption is indirect. For example, bicarbonate (HCO3−) does not have a transporter, so its reabsorption involves a series of reactions in the tubule lumen and tubular epithelium. It begins with the active secretion of a hydrogen ion (H+) into the tubule fluid via a Na/H exchanger:
In the lumen The H+ combines with HCO3− to form carbonic acid (H2CO3) Luminal carbonic anhydrase enzymatically converts H2CO3 into H2O and CO2 CO2 freely diffuses into the cell In the epithelial cell Cytoplasmic carbonic anhydrase converts the CO2 and H2O (which is abundant in the cell) into H2CO3 H2CO3 readily dissociates into H+ and HCO3− HCO3− is facilitated out of the cell's basolateral membrane
Influence of hormones Some key regulatory hormones for re-absorption include:
aldosterone, which stimulates active sodium re-absorption (and water as a result) anti-diuretic hormone, which stimulates passive water re-absorption Both hormones exert their effects principally on the collecting ducts. Tubular secretion occurs simultaneously during re-absorption of filtrate. Substances, generally produced by body or the by-products of cell metabolism that can become toxic in high concentration, and some drugs (if taken). These all are secreted into the lumen of renal tubule. Tubular secretion can be either active or passive or co-transport. Substances mainly secreted into renal tubule are; H+, K+, NH3, urea, creatinine, histamine and drugs like penicillin. Tubular secretion occurs at Proximal Convoluted Tubule (PCT) and Distal Convoluted Tubule (D.C.T); for example, at proximal convoluted tubule, potassium is secreted by means of sodium-potassium pump, hydrogen ion is secreted by means of active transport and co-transport, i.e. anti-porter, and ammonia diffuses into renal tubule.
Other functions
Hormone secretion The kidneys secrete a variety of hormones, including erythropoietin, calcitriol, and renin. Erythropoietin is released in response to hypoxia (low levels of oxygen at tissue level) in the renal circulation. It stimulates erythropoiesis (production of red blood cells) in the bone marrow. Calcitriol, the activated form of vitamin D, promotes intestinal absorption of calcium and the renal reabsorption of phosphate. Renin is an enzyme which regulates angiotensin and aldosterone levels.
Maintaining homeostasis The kidney is responsible for maintaining a balance of the following substances:
… excerpt ends here. Continue reading the full article.
